FIGS. 1 and 2 illustrate a conventional axial-flow fan 10 for heat dissipation. As shown in FIGS. 1 and 2, the axial-flow fan 10 comprises: a frame 12 with an air inlet 14 and an air outlet 16 respectively disposed at opposing top and bottom sides of the frame 12; a driving motor 18 mounted within the frame 12 for driving the fan 10 to operate; and a blade structure 20 connected to the driving motor 18. The blade structure 20 is composed of a hub 22 linked to and driven by the driving motor 18 to rotate, and a plurality of blades 26 peripherally mounted to the hub 22 and arranged vertically to an axial direction of the blade structure 20.
When the driving motor 18 of the fan 10 drives the blade structure 20 to operate, all the blades 26 on the hub 22 are adapted to rotate rapidly, allowing air to enter substantially at an axial direction into the fan 10 via the air inlet 14 of the frame 12, so as to generate airflow outputted substantially in an axial direction via the air outlet 16 of the frame 12 for use to help dissipate heat produced from an electronic device (not shown) mounted with the fan 10.
FIG. 3 illustrates a curve of pressure vs. quantity of airflow outputted from the axial-flow fan 10 operating under a predetermined rotating speed. As shown in FIG. 3, when the blade structure 20 of the fan 10 rotates at a predetermined speed, a particular PQ-curve 30 represents correlation between pressure (P) and quantity (Q) of airflow outputted from the air outlet 16. In other words, different PQ-curves are obtained for the fan 10 under different operating/rotating speeds. Thereby, the fan 10 can be adapted to operate under a desirably optimal condition according to the PQ-curve 30 and structural design of the electronic device, in an effort to achieve preferable heat dissipation performances for the electronic device.
However, in consideration of operating speed limits of the fan 10 driven by the driving motor 18 and axial flow direction of air into the fan 10, under a certain operating/rotating speed, the fan 10 may not be operatable under all conditions derived from the PQ-curve 30, and thereby may not attain to truly optimal efficacy for dissipating heat generated from the electronic device mounted with the fan 10.